Directional drilling involves controlling the direction of a borehole as it is being drilled. Since boreholes are drilled in three-dimensional space, the direction of a borehole includes both its inclination relative to vertical (dip) as well as its azimuth.
Usually the goal of directional drilling is to reach a target subterranean destination with the drill string, typically a potential hydrocarbon producing formation.
In order to optimize the drilling operation, it is often desirable to be provided with information concerning the environmental conditions of the surrounding formation being drilled and information concerning the operational and directional parameters of the downhole motor drilling assembly including the drilling bit. For instance, it is often necessary to adjust the direction of the borehole while directional drilling, either to accommodate a planned change in direction or to compensate for unintended and unwanted deflection of the borehole. In addition, it is desirable that information concerning the environmental, directional and operational parameters of the drilling operation be provided to the operator on a real time basis. The ability to obtain real time data measurements while drilling permits a relatively more economical and more efficient drilling operation.
For example, the performance of the downhole motor drilling assembly, and in particular the downhole motor, and the life of the downhole motor may be optimized by the real time transmission of the temperature of the downhole motor bearings or the rotations per minute of the drive shaft of the motor. Similarly, the drilling operation itself may be optimized by the real time transmission of environmental or borehole conditions such as the measurement of natural gamma rays, borehole inclination, and borehole pressure, resistivity of the formation and weight on bit. Real time transmission of this information permits real time adjustments in the operating parameters of the downhole motor drilling assembly and real time adjustments to the drilling operation itself.
Accordingly, various measurement-while-drilling (MWD) systems have been developed that permit downhole sensors to measure real time drilling parameters and to transmit the resulting information or data to the surface substantially instantaneously with the measurements. For instance, MWD mud pulse telemetry systems transmit signals from an associated downhole sensor to the surface through the drilling mud in the drill string. More particularly, pressure or acoustic pulses, modulated with the sensed information from the downhole sensor, are applied to the mud column and are received and demodulated at the surface. The downhole sensor may include various sensors such as gamma ray, resistivity, porosity or temperature sensors for measuring formation characteristics or other downhole parameters. In addition, the downhole sensor may include one or more magnetometers, accelerometers or other sensors for measuring the direction or inclination of the borehole, weight-on-bit or other drilling parameters.
Typically, MWD systems, such as the MWD mud pulse telemetry system, are located above the downhole motor drilling assembly. For instance, when used with a downhole motor, the MWD mud pulse telemetry system is typically located above the motor so that it is spaced a substantial distance from the drilling bit in order to protect or shield the electronic components of the MWD system from the effects of any vibration or centrifugal forces emanating from the drilling bit. Further, the downhole sensors associated with the MWD system are typically placed in a non-magnetic environment by utilizing Monel collars in the drill string below the MWD system.
Thus, the MWD system may be located a significant distance from the drilling bit. As a result, the environmental information measured by the MWD system may not necessary correlate with the actual conditions surrounding the drilling bit. Rather, the MWD system is responding to conditions that are substantially spaced from the drilling bit. For instance, a conventional MWD system may have a depth lag of up to or greater than 60 feet. As a result of this information delay, it is possible to drill completely through a potential hydrocarbon producing formation before detecting its presence, requiring costly corrective procedures.
In response to this undesirable information delay or depth lag, various near bit sensor systems or packages have been developed which are designed to be placed adjacent or near the drilling bit. The near bit system permits the detection of the potential hydrocarbon producing formation almost immediately upon its penetration, minimizing the need for unnecessary drilling and service costs. The drilling operation, including the trajectory of the drilling bit, may then be adjusted in response to the sensed information. However, in order to use a near bit sensor system and permit real time monitoring and adjustment of drilling parameters, a system or method must be provided for transmitting the measured data or sensed information from the downhole sensor either directly to the surface or to a further MWD system for subsequent transmission to the surface. Various attempts have been made in the prior art to transmit the information directly or indirectly to the surface. However, none of these attempts have provided a fully satisfactory solution.
Various systems have been developed for communicating or transmitting the information directly to the surface through an electrical line, wireline or cable to the surface. These hard-wire connectors provide a hard-wire connection from the drilling bit to the surface, which has a number of advantages. For instance, these connections typically permit data transmission at a relatively high rate and permit two-way or bidirectional communication. However, these systems also have several disadvantages.
First, a wireline or cable must be installed in or otherwise attached or connected to the drill string. This wireline or cable is subject to wear and tear during use of the system and thus, may be prone to damage or even destruction during normal drilling operations. For instance, the downhole motor drilling assembly may not be particularly suited to accommodate such wirelines running through the motor, with the result that the wireline sensors may need to be spaced a significant distance from drilling bit. Further, the wireline may be exposed to excessive stresses at the point of connection between the sections of drill pipe comprising the drill string. As a result, the system may be somewhat unreliable and prone to failure, which may result in costly inspection, servicing and replacement of the wireline. In addition, the presence of the wireline or cable may require a change in the usual drilling equipment and operational procedures. The downhole motor drilling assembly may need to be particularly designed to accommodate the wireline. As well, the wireline may need to be withdrawn and replaced each time a joint of pipe is added to the drill string. These disadvantages result in a relatively complex and unreliable system for transmitting the sensed information.
Systems have also been developed for the transmission of acoustic or seismic signals or waves through the drill string or surrounding formation. A downhole acoustic or seismic generator generates the acoustic or seismic signals. However, a relatively large amount of power is typically required downhole in order to generate a sufficient signal such that it is detectable at the surface. To generate a sufficient signal, the necessary power may be supplied to the generator by a hard wire connection from the surface to the downhole generator. Alternately, a relatively large power source must be provided downhole.
U.S. Pat. No. 5,163,521 issued Nov. 17, 1992 to Pustanyk, et al., U.S. Pat. No. 5,410,303 issued Apr. 25, 1995 to Comeau, et al., and U.S. Pat. No. 5,602,541 issued Feb. 11, 1997 to Comeau, et al. all describe a MWD tool, a downhole motor having a bearing assembly and a drilling bit. A sensor and a transmitter are provided in a sealed cavity within the housing of the downhole motor bearing assembly, adjacent the drilling bit. A signal from the sensor is transmitted by the transmitter to a receiver in the MWD tool. The MWD tool then transmits the information to the surface. The signals are transmitted from the transmitter to the receiver by a wireless system. Specifically, the information is transmitted by frequency modulated acoustic signals indicative of the sensed information.
Preferably, the transmitted signals are acoustic signals having a frequency in the range of from 500 to 2,000 Hz. However, alternatively, radio frequency signals of up to 3,000 mega-Hz may be used.
Further systems have been developed which require the transmission of electromagnetic signals through the surrounding formation. Electromagnetic transmission of the sensed information often involves the use of a toroid positioned adjacent the drilling bit for generation of an electromagnetic wave through the formation. Specifically, a primary winding, carrying the sensed information, is wrapped around the toroid and the drill string forms a secondary winding. A receiver may be either connected to the ground at the surface for detecting the electromagnetic wave or may be associated with the drill string at a position uphole from the transmitter.
Generally speaking, as with acoustic and seismic signal transmission, the transmission of electromagnetic signals through the formation typically requires a relatively large amount of power, particularly where the electromagnetic signal must be detectable at the surface. Further, attenuation of the electromagnetic signals as they are transmitted through the formation is increased with an increase in the distance over which the signals must be transmitted, an increase in the data transmission rate and an increase in the electrical resistivity of the formation. The conductivity and the heterogeneity of the surrounding formation may particularly adversely affect the propagation of the electromagnetic radiation through the formation. As well, noise in the drill string, particularly from the downhole motor drilling assembly, may interfere with the detection of the electromagnetic signals.
Thus, as with acoustic and seismic signal transmission, in order to be able to generate a sufficient electromagnetic signal, the necessary power may need to be supplied to a downhole electromagnetic generator by a hard wire connection from the surface. Alternately, a relatively large power source may be provided downhole.
Finally, when utilizing a toroid for the transmission of the electromagnetic signal, the outer sheath of the drill string must protect the windings of the toroid while still providing structural integrity to the drill string. This is particularly important given the location of the toroid in the drill string since the toroid is often exposed to large mechanical stresses during the drilling operation. Further, in order to avoid short-circuiting of the system or a short circuit turn of the signals through the drill string and in order to enhance the propagation of the electromagnetic radiation through the surrounding formation, an electrical discontinuity is provided in the drill string. The electrical discontinuity typically comprises an insulative gap or insulated zone provided in the drill string. An insulating material comprising a substantial area of the outer sheath or surface of the drill string may provide the insulative gap. For instance, the insulating material may extend for ten to thirty feet along the drill string.
Thus, the need for the insulative gap to be incorporated into the drill string may interfere with the structural integrity of the drill string resulting in a weakening of the drill string at the gap. Further, the insulating material provided for the insulative gap may be readily damaged during typical drilling operations.
Various attempts have been made in the prior art to address these difficulties or disadvantages associated with electromagnetic transmission systems. However, none of these attempts have provided a fully satisfactory solution.
U.S. Pat. No. 4,496,174 issued Jan. 29, 1985 to McDonald, et al. and U.S. Pat. No. 4,725,837 issued Feb. 16, 1988 to Rubin discloses an insulated drill collar gap sub-assembly for a toroidal-coupled telemetry system. The sub-assembly provides a dielectric material in the insulative gap, while attempting to enhance the structural integrity of the sub-assembly at the gap. Although the sub-assembly may enhance the structural integrity of the drill string, the system still requires the propagation of the electromagnetic radiation through the formation to the surface. Specifically, electromagnetic waves are launched from a transmitting toroid in the form of electromagnetic waves traveling through the earth. These waves eventually penetrate the earth's surface and are picked up by an uphole receiving system. The uphole receiving system comprises a plurality of radially extending arms of electrical conductors about the drilling platform, which are laid on the ground surface and extend for three to four hundred feet away from the drill site. These receiving arms intercept the electromagnetic waves and send the corresponding signals to a receiver.
U.S. Pat. No. 4,691,203 issued Sep. 1, 1987 to Rubin, et al. is directed at a downhole telemetry apparatus for transmitting electromagnetic signals to the surface. The apparatus includes a mode transducer designed to avoid the need for a toroidal transformer. The transducer provides a total electrical discontinuity in the drill string so that a potential difference can be produced across adjacent conducting faces of the drill string. Essentially, the adjacent conducting faces of the drill string are separated from each other by a predetermined insulative gap. Insulation around the gap is selected to induce optimum earth currents when the electrical signal is applied across the faces. Once the signal crosses the insulative gap, it is conducted to the surface through an upper portion of the drill string, where it is transferred from the drill string through a wire to an input transformer for a surface receiver. Once flowing through the transformed primary, the signal is transmitted through a wire installed in the ground near the surface. The electrical signal from the wire propagates through the earth back to the downhole sensor unit and finally completes its path into the mode transducer.
U.S. Pat. No. 5,160,925 issued Nov. 3, 1992 to Dailey, et al. and PCT International Application PCT/US92/03183 published Oct. 29, 1992 as WO 92/18882 are directed at a short hop communication link for a downhole MWD system. The system comprises a sensor module, a control module, a host module and a mud pulsar. The sensor module includes a transmitter for transmitting an electromagnetic signal, indicative of the information measured by the sensor, to the control module and a receiver for receiving commands from the control module. The control module includes a transceiver for transmitting command signals and receiving signals from the sensor module. Further, the control module transmits electrical signals to the host module through a hard wire connection, which similarly connects to the mud pulsar.
Both the sensor and control modules include an antenna arrangement through which the electromagnetic signals are sent and received through a short hop communication link. The sensor and control antennas are transformer coupled, insulated gap antennas. More particularly, communication between the sensor and control modules is effected by electromagnetic propagation through the surrounding conductive earth. The signal is impressed across an insulated axial gap in the outer diameter of the drill string, represented by the antennas, either by transformer coupling or by direct drive across a fully insulated gap in the assembly. The electromagnetic wave from the antenna propagates through the surrounding conductive earth, accompanied by a current in the metal drill string. As the formation conductance increases and resistance decreases, the maximum frequency with acceptable attenuation will decrease. Preferably, a frequency in the range of about 100 to 10,000 Hz is used.
U.S. Pat. No. 5,359,324 issued Oct. 25, 1994 to Clark, et al. and European Patent Specification EP 0 540 425 B1 published Sep. 25, 1996 are directed at an apparatus for determining earth formation resistivity and sending the information to the surface. The apparatus utilizes a first toroidal coil antenna mounted, in an insulating medium, on a drill collar for transmitting and/or receiving modulated information signals which travel through the surrounding earth formation. A second toroidal coil antenna is also mounted, in an insulating medium, on the drill collar for transmitting and/or receiving the modulated information signals to and from the first antenna.
More recent approaches have involved the use of special drill pipe equipped with data links. The disadvantages of this method include high cost associated with the special pipes and unreliability of the couplings in the joints.
Optic fiber has been used to provide a broadband telemetry system. U.S. Pat. No. 6,041,872 teaches an apparatus having a bared optic fiber cable stored in a spool. The spool can be fit into the drill string and thus the cable will not interfere with adding additional pipes. That attempt has failed because the naked optic fiber cannot withstand the harsh drilling environment. U.S. Pat. No. 6,655,453 records another attempt using armored fiber optic cable for telemetry purposes. Because of the limited space for the cable spool inside the drill string, cable diameter must be small in order to cover the entire borehole length. A thin cable, however, usually means a weak cable that may break in the harsh drilling environment.
As revealed above, there remains a need in the industry for reliable real time data telemetry systems and methods for communicating information between multiple positions in a wellbore. The proposed systems and methods of the present invention therefore, address the disadvantages or difficulties associated with conventional telemetry systems and methods.